A lactic acid oligomer having a cyclic structure is a useful compound which is used as a medicament such as a tumor cell growth inhibiting agent (Japanese Patent Application Laying-Open-(Kokai) No. 3-193731) or an antineoplastic agent (Japanese Patent Application Laying-Open (Kokai) No. 9-227388), or an intermediate thereof.
The conventional method for producing such a lactic acid oligomer involves subjecting lactic acids to dehydration condensation by heating under an inactive atmosphere, and then separating and collecting an oligomer component from the obtained reaction products.
However, since it is difficult to produce a lactic acid oligomer selectively by this conventional method and that the lactic acid polymer obtained in the dehydration condensation process of lactic acids has a broad molecular weight distribution, containing high polymers, it is necessary to separate and collect a lactic acid oligomer by separation means such as chromatography.